Recently, optical disc drives with both reading and writing capabilities, such as DVD-RAM recorders, for example, have been rapidly popularized. In writing data on a rewritable optical disc, the power of a laser beam (which will be referred to herein as the “laser power”) needs to be adjusted appropriately, which is one of the most important issues for rewritable optical disc technologies.
A recordable optical disc includes a data storage layer, which is made of a phase change material such as GeTeSb, for example, on a substrate. In writing data on such an optical disc, this data storage layer is exposed to a relatively high-power laser beam. Then, a portion of the data storage layer, which has been irradeated by the laser beam, comes to have a locally increased temperature, thereby changing the “phase” of the data storage layer due to the heat generated. More specifically, that laser-exposed portion of the data storage layer changes from the crystal phase to the amorphous phase. Such an amorphous portion of the layer will function as a recording mark. Accordingly, when the data writing process is completed, multiple recording marks and spaces will be alternately formed along the tracks. In this case, those recording marks have a different refractive index from the spaces. Thus, when the tracks on the data storage layer are irradeated by a laser beam that has too low an intensity to change the crystallinity of the data storage layer, data can be read out from the data storage layer according to the intensity of the reflected light. In this manner, when reading data from such an optical disc, the power of the laser beam should be defined weak enough to keep the crystallinity of the data storage layer unchanged.
However, various problems will arise unless the laser power to write data on such an optical disc (which will be referred to herein as “write power”) is controlled to an appropriate level. For example, if the laser power is too weak, then the exposed portion of the data storage layer will not be heated well enough to cause the phase change, and the information will not be written as intended. On the other hand, if the laser power is too strong, then the laser-exposed and heated portion will expand its area so much that a portion of the information previously recorded on an adjacent track might be cross-erased. To overcome these problems, an optical disc drive normally has the function of controlling the laser power to an appropriate level during a data write operation. When such an optical disc drive starts to work with an optical disc loaded thereto, the optical disc drive, first of all, carries out a self calibration, that is, a sequence of optimizing the write power. The best write power level is not the same among various types of the specific optical discs loaded.
On the other hand, not only the cross-erasing but other problems will not occur as long as the laser power to read data from such an optical disc (which will be referred to herein as “read power”) is weak enough to keep the crystallinity of the data storage layer unchanged. For that reason, even when the conventional optical disc drive starts with such an optical disc loaded thereto, the optical disc drive never carries out the sequence of optimizing the read power.
However, the present inventors discovered and confirmed via experiments that if the read power was not weak enough, then the temperature of the laser-exposed portion of the data storage layer slightly increased to reach the vicinity of the phase-changeable level (i.e., from the amorphous level to the crystal level). Accordingly, if a portion of the data storage layer with a recording mark is irradeated by the excessive reading laser beam, then the recording mark in the amorphous state might be recrystallized at least partially. In that case, the quality of the information stored there would deteriorate and sufficient read stability could not be maintained anymore. Such a decrease in read stability due to the exposure to that slightly intense reading laser beam will be referred to herein as “read light induced deterioration”. On the contrary, it should be noted that if the read power is too weak, then the readout signal cannot have a sufficiently high SNR and read errors are very likely to occur.
Japanese Laid-Open Utility Model Publication No. 63-062930 discloses a technique of performing a feedback control operation such that the modulated writing laser power is equalized with an average of binary information to be written.
Japanese Laid-Open Publication No. 06-139578 discloses a technique of optimizing the write power highly precisely by trying writing a test pattern of a particular signal.
Japanese Laid-Open Publication No. 05-250673 discloses a technique of saving the trouble of starting the same optimization search all over again by storing the result of such trial writing.
Japanese Laid-Open Publication No. 12-251266 discloses a technique of increasing the write stability by sufficiently decreasing the laser power while data is being read.
Each of these conventional techniques was developed on the assumption that the laser power always be detected accurately. Also, each of these techniques requires that a feedback control be done to control the laser power detected at a predetermined target value.
Recently, however, to increase the storage densities of optical discs, short-wavelength laser light sources have been developed and gradually adopted in actual optical disc drives. For example, the conventional red laser light source is going to be replaced by a blue laser light source. This trend will be more and more accelerated because the shorter the wavelength of a laser beam emitted, the smaller the size of the laser beam spot on the optical disc.
However, the shorter the wavelength of the laser beam is, the higher the energy of light impinging on a photodetector for detecting the laser power becomes. The energy of light per photon is given by e=hυ, where e is the energy per photon, h is Planck's constant, and υ is the frequency of light. υ is inversely proportional to the wavelength of the light. Thus, it can be seen that the shorter the wavelength, the higher the energy e.
Also, such high-energy light will give more and more damage on optical elements, which are usually made of crystals or polymers. For example, a PIN photodiode, which is often used as a photodetector, will gradually decrease its sensitivity when exposed to a blue ray for a long time.
Furthermore, as the sensitivity of such a photodetector decreases, the controlled laser power also varies, which is a non-negligible problem. In the conventional techniques mentioned above, the laser power control system carries out a feedback control so that the output (current value) of the photodetector gets equal to a target value. Accordingly, once the sensitivity of the photodetector has decreased, the output (current value) thereof will also decrease even if the photodetector has received the laser beam of the same intensity. As a result, the feedback control works such that the output (current value) of the photodetector approaches the target value. Consequently, the laser power will increase beyond its original level.
In the technique of finding the best write power to obtain the best readout signal while sequentially changing the write power and actually writing test data on the optical disc, the write power can still be optimized even if the sensitivity of the photodetector has decreased. This is because this technique enables the target value to be controlled, thereby redefining the output of the photodetector at a lower value once the sensitivity of the photodetector has decreased.
The conventional optical disc drives may sometimes optimize the write power but none of those optical disc drives has ever been designed to control the read power. This is because nobody has ever believed that the deterioration in read stability, which should be caused by the increase in read power when the sensitivity of the photodetector decreases, would bring about a serious problem. Accordingly, in the conventional optical disc drives, when the sensitivity of the photodetector decreases in a long-time exposure, the data will be read out with a laser beam that has so high a level as to crystallize the recording marks partially, thus also resulting in the read light induced deterioration.